Heretofore the methods for detecting tone signals having selected frequencies have utilized bandpass predetection filters. A high "Q" detector of this type requires a multipole filter network in which both the center frequency and the filter roll off characteristics must be highly stable and closely controlled. With multipole networks of this type it is very difficult to reprogram the center frequency. Further, it is difficult to track a dynamic signal which has frequency variations caused by Doppler shift or by other effects.
In view of these limitations there exists the need for a tone detector which has frequency stability, is highly selective, can be easily reprogrammed to detect a wide range of frequencies and further includes the capability to track a dynamic signal over a range of frequencies.